The fluidized catalytic cracking of hydrocarbons is the main stay process for the production of gasoline and light hydrocarbon products from heavy hydrocarbon charge stocks such as vacuum gas oils or residual feeds. Large hydrocarbon molecules, associated with the heavy hydrocarbon feed, are cracked to break the large hydrocarbon chains thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed.
The basic equipment or apparatus for the fluidized catalytic cracking of hydrocarbons has been in existence since the early 1940's. The basic components of the FCC process include a reactor, a regenerator and a catalyst stripper. The reactor includes a contact zone where the hydrocarbon feed is contacted with a particulate catalyst and a separation zone where product vapors from the cracking reaction are separated from the catalyst. Further product separation takes place in a catalyst stripper that receives catalyst from the separation zone and removes entrained hydrocarbons from the catalyst by counter-current contact with steam or another stripping medium. The FCC process is carried out by contacting the starting material, whether it be vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbons, with a catalyst made up of a finely divided or particulate solid material. The catalyst is transported like a fluid by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport. Contact of the oil with the fluidized material catalyzes the cracking reaction. The cracking reaction deposits coke on the catalyst. Coke is comprised of hydrogen and carbon and can include other materials in trace quantities such as sulfur and metals that enter the process with the starling material. Coke interferes with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place. Spent catalyst, i.e., partially deactivated by the deposition of coke upon the catalyst, exits the reactions zone. Traditionally, catalyst passes from the stripper to a regenerator that removes the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content, relative to the catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. Some of the heat may also be recovered by heat exchange of a circulating catalyst stream against a cooling fluid such as boiler feed water to generate steam. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst circulates continuously from the reaction zone to the regeneration zone and then again to the reaction zone. The fluidized catalyst, as well as providing a catalytic function, acts as a vehicle for the transfer of heat from zone to zone. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
The rate of conversion of the feedstock within the reaction zone is controlled by regulation of the temperature of the catalyst, activity of the catalyst, quantity of the catalyst (i.e., catalyst to oil ratio) and contact time between the catalyst and feedstock. The most common method of regulating the reaction temperature is by regulating the rate of circulation of catalyst from the regeneration zone to the reaction zone which simultaneously produces a variation in the catalyst to oil ratio as the reaction temperatures change. That is, increase in the flow rate of circulating fluid catalyst from the regenerator to the reactor effects an increase in the conversion rate. Since the catalyst temperature in the regeneration zone is usually held at a relatively constant temperature, significantly higher than the reaction zone temperature, any increase in catalyst flux from the relatively hot regeneration zone to the reaction zone raises the reaction zone temperature.
One improvement to FCC units, that has reduced the product loss by thermal cracking, is the use of riser cracking. In riser cracking, regenerated catalyst and starting materials enter a pipe reactor. The expansion of the gases formed by contact with hot catalyst upon the hydrocarbons and other fluidizing mediums, if present, transports the mixture upward. Riser cracking provides good initial catalyst and oil contact and also allows the time of contact between the catalyst and oil to be more closely controlled by eliminating turbulence and backmixing that can vary the catalyst residence time. An average riser cracking zone today will have a catalyst to oil contact time of 1 to 5 seconds. A number of riser designs use a lift gas as a further means of providing a uniform catalyst flow. Lift gas accelerates catalyst in a first section of the riser before introduction of the feed and thereby reduces the turbulence which can vary the contact time between the catalyst and hydrocarbons.
In most reactor arrangements, catalysts and conversion products still enter a large chamber that initially disengages catalyst and hydrocarbons. Cyclone separators use centripetal acceleration to disengage the heavier catalyst particles from the lighter vapors and to perform a final separation of hydrocarbon vapors which exit the reaction zone.
Product recovery facilities recover the hydrocarbon product of the FCC reaction in vapor form. These facilities normally comprise a primary fractionation zone, more commonly referred to as the main column for cooling the hydrocarbon vapor from the reactor and recovering a series of heavy cracked products which usually include bottoms material, cycle oil, and heavy gasoline. Lighter materials from the main column enter a concentration section for further separation into additional product streams.
The advances in FCC technology have enabled FCC unit owners to increase the feed process in a given size unit. These advances include changes to the internals of the FCC unit as well as new catalyst developments and the use of other additives to increase the feed processing capacity of an FCC unit. The ability to increase the feed to the FCC unit can be limited by the size of the product separation facilities associated with the FCC unit which create a bottle-neck for further increases in the processing capacity. In particular, it has been found that the gas concentration section and more particularly the wet gas compressor, which is a main source of energy for the gas concentration section, will limit the total throughput of the FCC unit. The use of lift gas in an FCC unit can compound the difficulties in increasing throughput due to additional recycling of gas through the gas concentration section back to the reaction section.
While the benefits of using lift gas to pre-accelerate and condition regenerated catalyst in a riser type conversion zone are well known, lift gas typically has a low concentration of heavy hydrocarbons, i.e. "wet" gas comprising propane and higher boiling hydrocarbons, are usually avoided. Thus the recycling of lift gas streams, comprising a large quantity of dry gas (i.e., hydrocarbons having a lower boiling point than propane) impose extra burdens on the wet gas compressor of the gas concentration section by taking up capacity in the compressor which could be used for wet or heavy hydrocarbons.